The proposed project is concerned with understanding the molecular basis of differential gene expression during development. The rationale for the experimental approach is that the embryonic and adult isozymes of L-glycerol 3-phosphate dehydrogenase represent a specific example of differential gene expression, since first the gene for the embryonic isozyme and then later the gene for the adult isozyme dominate in the cell in determining a specific biochemical function. Previous results have demonstrated that this isozyme transition occurs in reaggregating cell cultures of mouse cerebellar cells, and that the accumulation of enzyme activity of the adult isozyme in culture is accompanied by an increase in enzyme synthesis as measured by the incorporation of isotopically labelled amino acid into the enzyme. Future experiments will focus on determining whether increases in enzyme synthesis is accompanied by an increase in specific messenger RNA and on analyzing the apparent need of a three-dimensional tissue culture before isozyme transition and the accumulation of the adult isozyme occurs. In addition, morphological and ultrastructural studies suggest that reaggregating cell cultures of cerebellar cells may provide an excellent system for the study of neuronal and glial interactions, particularly as they apply to the pathological condition, fibrous gliosis.